Vulcanized rubber-silica composition



p 0, 1957 E. M. ALLEN 2,806,012

VULCANIZED RUBBER-SILICA COMPOSITION Original Filed April 22, 1952 INVENTOR. [M620 02 Au! ATTORNEY Unite 2,806,012 VULCANIZED RUBBER-SEICA COMPQSITION Original application April 22, 1952, Serial No. 283,721. Divided and this application March 23, 1957, Serial No. 640,133

6 Claims. (Cl. zen-41.5

This invention relates to a novel rubber composition comprising a vulcanized mixture of rubber (including synthetic rubber) and a precipitated siliceous pigment which has unusual strength properties.

Silica prepared by precipitation from aqueous solutions of alkali metal silicate has not heretofore been used suc cessfully on a commercial scale as a rubber reinforcing pigment. While certain relatively coarse silica powders and certain silica gels have been proposed for use in rubber, they have served primarily as fillers or diluents rather than as efiective reinforcing pigments.

According to the present invention, vulcanized silicarubber compositions having good strength properties have been prepared. When GR-S rubber has been used as the rubber component, vulcanized rubber products having tensile strengths above 2400 pounds per square inch have been produced. Natural rubber products of even higher strength can be produced due to the inherent high strength of natural rubber. When this silica is prepared by mixing the precipitated silica before drying with rubber latex and coagulating the mixture, the strength properties of the product obtained are at an optimum. However, good rubber can be produced in which the silica is Well dispersed, and which is free from pinholes or white specks, by drying the silica herein contemplated and then milling the dried product into the rubber.

The novel composition herein contemplated is a vulcanized mixture of rubber and a finely divided, precipitated silica in the form of porous fiocs, the silica containing in excess of 90 percent, preferably 94 percent or more, by weight of SiOz, measured on the anhydrous basis (that is, on a basis excluding bound and free water), bound water in the proportion of about 3 to 9 moles (normally about 6 moles) of SiO2 per mole of bound water, and from 2 to 10 percent of free water by weight of the pigment, and which has a surface area of about 60 to 200 square meters per gram, preferably in the range of 75 to 175 square meters per gram, and an average ultimate particle size in the range of 0.015 to 0.05 micron. Preferably, this pigment also should contain no more than 1.75 percent, generally less than 1 percent, by weight of N320, and may even be free of NazO. The surface area of the pigment may be measured by the Brunauer- Emmett-Teller method which is described in Journal American Chemical Society, vol. 60, page 309 (1938).

An especially valuable pigment of the type described above is one which contains a small amount of a metal of the group consisting of alkaline earth metals, such as calcium, magnesium, barium or strontium, and zinc. Very superior pigments have been prepared according to this invention which contain one mole of the metal oxide or oxides per 10 to 150 moles of SiOz. This metal is present as the metal oxide, possibly in chemical association with this silica, and its percentage concentration normally ranges from 0.5 to 6 percent, computed as the metal oxide, based upon the weight 'of the entire pigment.

Sttes atet The presence of the water and a metal, such as calcium, in the pigment appears to impart especially advantageous properties. The herein described silca has been determined to be safe from a health standpoint. This is understood to be due, among other factors, to the presence of these agents.

The above described silica compositions are useful reinforcing pigments in various rubber compositions, including natural rubber, and synthetic rubber compositions, including butadiene-1,3-styrene copolymers, butadi-' ene-acrylonitrile copolymers, butadiene-isobutylene copolymers (butyl rubber), and lilse'synthetic elastomers which are derived from polymerization of butadiene-1,3, Z-chlorobutadiene, isoprene, ethylene or the like, alone or with other polymerizable materials, including styrene,

methyl methacrylate, methyl chloroacrylate, acrylonitrile, vinyl chloride, and their equivalents.

Approximately 5 to parts by Weight of silica is incorporated per 100 parts by weight of the rubber. Best results are obtained when 40 to the silica is used per 100 parts of rubber. The rubber composition contains other conventional components, such as accelerators and modifying agents, such as listed in some of the ensuing examples. Temperatures of cure and methods of vulcanizing or milling these compositions are conventional and well understood in the art.

The problem of preparing a precipitated pigment of the type herein contemplated is quite diflicult. It is believed that none of the processes which are disclosed by the prior art produce a pigment of the type which has been provided according to the present invention.

The various factors which have been set forth above appear to be of considerable importance in the provision of a pigment which will elfectively reinforce rubber, including synthetic rubbers. The silica concentration of the pigment should be high and should exceed 90 percent by weight, frequentlybeing as high as 94 percent by weight or even higher, on the anhydrous basis.

As previously explained, the silica pigment used according to this invention appears under high magnification to be in the form of fiocs or aggregates of individual particles. These flocs are porous and resemble upon magnification a bunch of grapes; The average ultimate particle size of the silica is estimated by comparison to be about 0.02 to 0.03 micron.

Two types of water are present in my novel pigment. These types are termed bound water and free water. the term free water, as used herein, is intended to denote the water which may be removed from the silica pigment by heating the pigment at a temperature of C. for a period of 24 hours in a laboratory oven. The term bound Water, as used herein, is intended to mean the amount of water which is driven off from a silica pigment by heating the pigment at ignition temperature, for

example, 1000 to 1200 C., until no further watercan be removed, minus the amount of free water in the pigment. 7 a

It has been found, according to the present invention, that the presence of free water is advantageous. 'That is, pigments which do not contain freewater produce rubber compositions which cure relatively slowly un1essspecialv additives, such as ethylene glycol, are introduced. into thecomposition. H

It will be understood that the amount of water vremaining in a precipitated pigment depends upon the time, temperature, and other conditions of drying. .Thus, it is not possible to express the conditions which willbe required for drying aparticular pigment with. any degree of exactness. This, of course, will vary to a large degree dea 7 pending upon the degree of. air circulating'through the P e t, e yp o d y appara us-m m i e- -Tll l'ld V 80 parts by weight of the silica particle. For this reason, the bound water: does not come olf readily unless dried at temperatures above 350 C. 'Onfthe other hand, the free water comes off readily upon drying at'normal temperatures. Some portionof this water will be picked up on standing in atmospheric air of normal humidification. When bound Water is removed, only a small portion thereof is reabsorbed on standing. w T

The pigments herein contemplated may be prepared by a large number of methods. A particularly effective method of preparing the silicapigment herein. contemplated involves reaction of finely divided alkaline earth metal silicatesuch as calcium silicate having an average ultimateparticle sizebelow 0.1 micron, with an acid having an anion which forms a water soluble salt with the alkaline earth metal;

In the practice of this process, the acid is reacted with the calcium silicate in an aqueous medium, and sufi'icient acid is-added to largely decompose the calcium silicate,

' to extract the calcium therefrom, and to prevent establishment'of a concentration of calcium in the silica above about 6 percent by weight of the silica pigment, computedas CaO. Consequently, suificient acid normally is used to reduce the pH below about 5, usually in the range of 3to 5. During the acidification, the slurry of calcium silicate may be agitated in order to promote and facilitate reaction, In order to avoid use of excess acid, acid is added in small portions until the desired pH has been reached, as indicated for example by suitable indicators, such as methyl orange. In general, additions of large excesses'of acid beyond a pH of 0, for example, are unnecessary. After'the reaction of calcium silicate with acid-has been completed and the pH of the aqueous slurry has been reduced below about 4 or 5, the precipitated silicais recovered.

This silica, of course, is'present in an aqueous slurry. Serious problems arise in the recovery of the silica from the slurry by virtue of the fact that the silica fails to settle rapidly and also filters with extreme slowness. What'is even more important, the finely divided pigment thusobtained normally has a surfacearea well above 200 square meters per gram. 7 7

According to this invention, it has been found that thesedifiiculties may be avoided by readjusting the pH of the slurry above 5, generally in the rangeof 5 to 9. This adjustment is advantageous in order to facilitate separatio n of thefsilica from the aqueous medium. It is also, advantageous since it has a' material effect upon the surface area of the pigment produced and thereby insures that this pigment will have the surface area desired.

While more rapid filtration is achieved when the pH ofthe silica is 7 or above, pigments having an acid pH disperse better in rubber. For this reason, it is advantageous for the'silica to be slightly acidic. On 'the other hand, 'the'pH of the slurry should be maintained above about 5 to facilitate filtration. Best pigment. is achieved when the slurry is adjusted to above about 5 and below 7. In such a'case, the'pH of the pigment is about 6 to 8.5. While addition of excess acid followed by readjustment is found preferable, it is also possibleto add the acid directly to the slurry until the desired pHrange is achieved-without readjustment.

Following this, the pigment is recovered-'bysettling and/ or filtration. Thereafter, the pigment is'dn'ed.

In general,.alkali is added to the precipitated silica before the silica is completely separated from its mother liquor which containsdissolved calcium chlorideor like calcium salt... Thus,.the process normallyis conducted by adding acid to a slurry or suspension of the calcium silicate until the pH thereof is reduced to 5 .or below and, thereafter, alkali such as sodium hydroxide or like alkali 4 metal hydroxide is added to the resulting slurry. As a consequence, the silica contains an appreciable amount, usually 1 to 5 percent by weight, of CaO. After the adjustment of the slurry is eflFected, the silica is recovered.

In drying the pigment, care is exerted to avoid dehydration of the pigment to the point where all of the free water, as herein defined, is removed. Hence, the drying operation normally is discontinued while a substantial quantity of free water remains. The preferred ranges are as specified above. To elfectthis result, the temperature of drying should be neither too high nor too low. Furthermore, drying should not be continued over an excessively long period. In general, the temperatureof drying will range between 100200 0, usually in the range of about 120l40 C. The time of drying, of course, will depend upon the amount of water entrapped in the pigment and the amount of air circulating through or over the pigment. Periods of time ranging from 10 to 12 hours normally are satisfactory when the drying temperature is within the range of 120-140 C. and the drying is conducted in a stationary shelf dryer or aircirculating oven. Shorter or longer times may be necessary if. the. temperature of heating becomes higher or lower or if other drying equipment is used.

For special purposes as, for example, in silica grease or in silicone rubber or the like, it is advantageous to use a silica having a pH of 2 to 3.5. Such a product is difiicult to filter and requires special equipment to resist corrosion. In such a case, recovery of the silica from the slurry by spray drying is advantageous.

In order to obtain a product which has maximum pigmentary reinforcing characteristics when used in rubber compositions, it is necessary to use a special form of calcium silicate or similar alkaline earth metal sihcate. That is, some calcium silicates will not produce the results desired. In general, in order to obtain a proper pigment of the desired particle size from calcium silicate, it is necessary to use a precipitated calcium silicate having a surface area of 50 to 125 square meters per gram and an average ultimate particle size below 0.1 micron, usually 0.02 to 0.06 micron.

The manner in which the calcium silicate has been prepared has a definite influence upon the character of the silica which is obtained therefrom. Thus, is has been found that tensile tests of rubber compositions containing silica obtained from calcium silicate prepared by reaction of calcium acetate with sodium silicate were lower than those prepared as hereinafter described, either because they cured with extreme slowness or for other reasons.

The best silica which has been prepared from calcium.

silicate" has been obtained when the calcium silicate has been prepared by reacting calcium chloride with alkali metal silicate in aqueous medium containing sodium chloride or like alkali metal chloride. This sodium chloride conveniently may be in the calcium chloride solution although it may also bein the sodium silicate solution. Thus, it is found most desirable to react aqueous sodium silicate with an aqueous calcium chloride solution containing sodium chloride preferably in the proportion of at least 0.1 pound, and usually in the range of 0.2 to 0.5 pound, of sodium chloride per pound of calcium chloride. Normally, the NaCl content of the solution should be in excess -of.2 to 5 grams per liter. Solutions which contain higher sodium chloride content may be used. However, it israre that the Weight of sodium chloride will exceed the weight of calcium chloride in the solution or.

to 88 percent SiOz.

was 5 or grams per liter. Best pigments were obtained in such a case when the calcium chloride solution contained at least grams per liter, usually in the range of 50 to 150 grams per liter, and using sodium silicat solution containing in excess of 20 grams of SiOz per liter, usually in the range of 50 to 150 grams per liter of SiOz. More concentrated solutions containing up to about 200 grams per liter of CaClz and of SiOz or even higher may be used, although best results have been obtained when the concentration of the CaClz and SiO2 solutions is be low 200 grams per liter.

The proportion of calcium chloride solution to sodium silicate normally is suificient to react with all or at least most of th sodium silicate. In general, the amount of calcium chloride is in stoichiometric excess. However, small excesses of sodium silicate are not objectionable. Thus, it is possible to use sodium silicate 10 to percent in excess of the calcium chloride although best results are obtained when the calcium chloride is at least in stoichiometric amount. Excesses of sodium silicate as high as 100 percent over stoichiometn'c usually give unsatisfactory products. However, even such amounts may be used if the sodium chloride concentration is sufliciently high and the rate of acidification is held within the proper limits. Thus, the adverse effects of excess sodium silicate may be counteracted to an appreciable degree by the presence of sodium chloride in the reaction mixture subjected to acidification.

The precipitation of calcium or other alkaline earth metal silicate in finely divided state, such as is herein required, may be accomplished with best results by mixing a stream of aqueous sodium silicate solution with the cmcium chloride solution under conditions which subject the mixture to a high degree of turbulence and almost instantaneous mixing. The amount of reactants in the respective streams is proportioned so as to obtain calcium silicate in the desired concentration and to establish an excess of calcium chloride over the stoichiometric quantity required to react with the silicate. One effective way to produce the required turbulence is to introduce the two streams closely together into a central area of a centrifugal pump. In this case, the agitation of the mixture is effected as the introduced streams of the reactants are thrown radially outward by the pump rotor. In most cases, it is found desirable to limit the feed of the calcium chloride solution and alkaline metal silicate solutions to the pump to an amount below the capacity of the pump. For example, if the pump is capable of discharging 100 gmlons per minute with unlimited flow of liquid to the pump, the amount of reacting solution supplied to the pump is held at least 10 percent below, and usually percent or more below this amount. This appears to afford a greater degree of agitation of the reacting solutions and to ensure production of calcium silicate having the desired fineness.

To ensure production of the calcium silicate in a highly finely divided state, alkaline metal silicate having the composition Na2O(SiO2)I, where x is a number not less than 2 nor more than 4, is preferably used. This results in the production of a calcium silicate having the composition CaO(SiQZ)I, where x is as defined above. However, other calcium silicate, where x is higher, may be used in certain cases.

The resulting silica is a dry powder which is found to be in an extremely fine state of division and is preponderantly silica. By analysis the dried product normally contains above percent SiOz, the usual range being about 78 On the anhydrous basis the silica concentration of the product is above percent, usually being in excess of about percent. The surface area of this product ranges between 75 and 125 square meters per gram, as measured by the Brunauer-E-mmett-Teller method of determining surface area.

The pigment contains approximately 10 to 15 percent water. The free water content normally ranges between 2 to 10 percent, the balance being bound water.

The pigment prepared according to this method normally contains an appreciable concentration of calcium. This calcium content usually ranges between /2 to 6 percent, computed as calcium oxide. Because of this calcium content, the pH of the pigment is stabilized on the alkaline side. Other impurities, frequently in the range of 0.1 to 2 percent, such as iron and aluminum oxides, sodium chloride, and carbon dioxide, usually are present.

The following are typical analyses of silica samples made from various runs in which calcium silicate prepared as described above is reacted with hydrochloric acid as above described:

TABLE I Percent 7 Percent R20 Percent Percent Percent Sample No. SiOi (Aluminum 020 C1 11 0 and iron oxide) Percentages in the above table are by weight.

The acid used to effect the neutralization or decomposition of calcium silicate normally is hydrochloric acid. On the other hand, other acids which form water soluble anions with calcium may be used. Such acids include hypochlorous acid, hydrobromic acid, nitric acid, nitrous acid, and acetic acid. The following is an example of this process:

' Example I Streams of aqueous sodium silicate solution containing grams per liter of SiOz as Na2O(SiOz)3.3s, and calcium chloride solution containing 100 grams per liter of CaClz and 30 to 40 grams per liter of sodium chloride, were fed directly into the central area of a centrifugal pump at 150 F.

The rates of flow were adjusted so that calcium chloride was approximately 10 percent in excess over the stoichiometric quantity required for reaction, and so that the amount of liquid supplied to the pump was about 25 percent below the output capacity of the pump. In consequence, the solutions were subjected to turbulent intermixing in the pump. 7

The slurry of calcium silicate thus produced was introduced into a tank and sufficient hydrochloric acid solution containing 28 percent by weight of HCl was added, with stirring, to reduce the pH of the slurry to 2. Thereupon, sufiicient sodium hydroxide solution containing 40 percent by weight of NaOH was added to raise the pH of the slurry to 7.5 The precipitated silica was recovered by decantation and filtration, and was dried in an oven at a drying temperature of to C. for 12 hours. The free water content of the product was within the range of 3 to 8 percent by weight of the pigment.

It will be noted that the silica pigments may be prepared from materials other than calcium silicate. Thus,

' finely divided, precipitated magnesium silicate, barium silicate or strontium silicate, as well as silicates of zinc and other metals of series 3 to 8, group II of the periodic table, which have the surface area properties roughtly approximating those set forth with respect to calcium silicate, may be subjected to treatment with water soluble acids according to this invention in order to extract the metals and produce the herein contemplated pigment. In such a case, the magnesium or like silicate preferably is prepared as described above by reaction of metal chloride solution containing at least 0.1 pound of sodium chloride per poundof metal chloride.

The surface area of the resulting silica is det erinined by slurry and not of the dried pigment was obtained.

if suflicient acid is added to the calcium silicate to reduce the pH-to'as low as 2,for example, the silica which is thus obtainedhas an unusually high surface area. Such a'piginent, if recovered fromsuch slurry, would not be suitablefor the applicants purpose. On the other hand,

' whenthis slurry is treated with alkali to increase the pH to-above the surface area reduces as the pH increases, so that when the pH is '5 or above, the surface area has fallen to approximately 135.

a. The following example illustrates this principle:

. V Exainple II 7 Eight litersof calcium silicate slurry containing 100 grams per;1iter of calcium silicate, and prepared according to the method described in ExampleI, was placed in a l2-liter flask fitted with a stirrer. The slurry was heated to 70 C. while being stirred, and then hydrochloric acid solution having a strength of 3.5 normal was added at a rateof 100 milliliters per minute for 12 /2 minutes. Fifteen'minutes, after 'all the acid was added, 3.5 normal sodium hydroxide solution was added to the slurry, with agitation, at a rateof 100 milliliters per minute for ll minutes. During the addition of the hydrochloric acid and the sodium hydroxide, 100 milliliters of samples of slurry were withdrawn at the time intervals indicated in the table below, and placed in 4-ounce sample bottles which were then closed. These samples were allowed to stand for 3 days. Thereafter, the pH of the slurry samples was measured and the slurries filtered on a 100-milliliter Buchner funnel. The time was noted when the filter'cake lost its shine prior to cracking. This time interval was taken as the filtration time.

, Following filtration of the slurry, the filter cake was washed with distilled wa-ter until free of chloride ions.

The washed pigment samples were then dried in an oven at 105 C., ground in a mortar, and the surface area of the samples was measured.

The results obtained are summarized in the following table: a

TABLE II Filtration Surface time Area of Time Minutes Slurry (Minutes product pH and (Square Seconds) meters pergram) 15 minutes elapsed time the above tests, it will be noted that the pH of the generahit is found that. when thedriedpigment is slurried inwatenthe pH thereof is somewhat higher than that of equation:

the initial slurry. With slurries having a pH above about 8, this dilference' is only minor. On the other hand, with'slurries having-a pH below 8, the pH of the dry pigment, when reslurried; usually is'as much as 1 or 2 pH units above that of the pH of the initial slurry.

The above tests clearly indicate the effect of the pH upon the filtering characteristics andalso the surface area of the ultimate product. 'Thus, it is usually desirable to effect the reaction under conditions such that the pH of 'the slurry' prior'to filtration is in excess of 5 and, to achieve maximum dispersion in rubber, the slurry pH should be between about 5 and 7. V

'The'pigments contemplated within the scope of the present invention may beprepared by other methods. For example, the calcium silicate prepared as described above, and7or having the properties described above, may be reacted with an aqueous solution'of ammonium chlo-' ride. In such a case, the ammonium chloride reacts with the calcium silicate, liberating ammonia and precipitating silica. The reaction proceeds according to the following ZNHcCl-i-CEIO' (SiO2)I CaClz+2NHs+xSiOz+HO The reaction of calcium silicate with ammonium chloridemay be effected by adding the ammonium chloride,

usually 'as an aqueous solution, to an aqueous slurry of the calcium silicate. During this addition, the slurry is generally [agitated and, ultimately, the'slurry is heated to drive off the ammonia. Usually, this heating is continued until all or substantially all of the free ammonia has been driven on, Sufiicient ammonia is used to ensure decomposition of substantially all of the calcium silicate.

Advantageously, the ammonium chloride solution should contain an appreciable amount of ammonia and sodium chloride. Silica which has especially valuable properties has been prepared using such a solution. The ammonia serves, among other purposes, to maintain the pH of the slurry within a range at which a readily filterable slurry is obtained. The amount of ammonia and sodium chloride which may be present is capable of considerable variation. Solutions containing in excess of 5, and usually ranging from 5 to 50 grams per liter of free ammonia, are found to be suitable. The sodium chloride content of solutions used according to this invention normally exceeds 25 grams per liter of solution, usually being in the range of 50 to grams of NaCl per liter of solution.

After reaction of the calcium silicate with the ammonium chloride is completed and the resulting free ammonia has been removed, the resulting slurry is treated to recover the pigment suspended therein. This may be done effectively by conventional decantation and/orfiltration operations. In the course of this operation, water soluble salts, such as calciumchloride, ammonium chloride, etc., are washed from the pigment and the resulting product is dried at a suitable temperature.

Some improvement in pigment properties of silica produced by reaction of calcium silicate with ammonium chloride is obtained when the resulting silica is subjected to the reaction of acid after removal of ammonia is es- These acids should be capable of Following this acid treatment, it frequently Will be advantageous. to neutralize excess acidity and to ensure production of a pigment having a pH above 5. The following examples are illustrative: V

' Example Ill An aqueous solution of sodium silicate was prepared by diluting 5.88 liters of sodium silicate containing298 grams per ,liter of 8102: 215 sodium silicate having the 9 composition'Na2O(SiO2)s.3e, with sufiicient water to produce 20.7 gallons of solution. Afurther solution was made by dissolving 1220 grams of calcium chloride and 320 grams of sodium chloride in 16.0 gallons of water. Streams of these aqueous solutions were fed directly into the central area of a centrifugal pump, proportioning the rates of flow so that calcium chloride remained in excess over the stoichiometric quantity required for reaction with the sodium silicate at all times. After mixing of the two solutions was complete, 475 grams of ammonium chloride was added to the resulting calcium silicate slurry and the slurry was thereafter boiled for about 4 hours, at which time the odor of ammonia was very faint. Thereafter, the slurry was washed and filtered, and was dried at a. temperature of about 120 C. A white friable product having the following composition was produced:

Percent by weight Ignition loss 13.67 SiOz 77.84 R203 (iron and aluminum) 1.30 CaO 5.88 MgO 1.36

Example IV 47.1 liters of sodium silicate solution containing 298 grams per liter of SiO2 as Na2O'(SiO2)3.3s was diluted to 145 gallons. 87.5 gallons of an aqueous solution containing 10,650 grams of calcium chloride and 2,800 grams of sodium chloride was made up. These solutions were mixed with vigorous agitation as in Example III. The slurry precipitate was washed to remove dissolved chlorides, and an aqueous slurry containing 42.7 grams of calcium silicate per liter of slurry was obtained. Fifty gallons of this calcium silicate slurry was mixed with 23.19 liters of aqueous ammonium chloride solution prepared from the preparation of soda ash according to the Solvay process by reaction of sodium chloride, ammonia, and carbon dioxide, in aqueous solution. This solution contained 160 grams per liter of NHaCl and about 20 grams per liter of free ammonia, together with about 70 grams per liter of NaCl.

The resulting mixture was heated to boiling until no further ammonia was given off. Thereafter, the precipitate was filtered, washed, dried, and pulverized. The resulting product is preponderantly SiOz and is a useful rubber pigment. It was compounded with a GR-S composition according to the following formula:

Percent by weight GR-S 100 Zinc oxide Sulphur 3 n-Cyclohexyl 2-benzothiazol sulphenamide 1 Processing oil 5 Phenyl beta naphthyl amine l Silica as prepared above 68.2

Example V Calcium silicate slurry was prepared according to the process generally described in Example I, using an aqueous solution of sodium silicate containing 100 grams per liter of SiOz as NazO(SiO2)3.3e, and calcium chloride solution containing 100 grams per liter of CaClz and 30 to 40 grams per liter of NaCl at a temperature of 150 F. The resulting slurry of calcium silicate contained 0.35 gram equivalents per liter 'of alkalinity, as determined by titration with HCl to methyl orange end point.

One hundred gallons of this calcium silicate slurry was placed in a tank and mixed with 20 gallons of ammonium chloride solution containing 1.95 gram equivalents of NhtCl per liter of solution as well as 15 grams per liter of free ammonia and 70 grams per liter of NaCl.

The resulting slurry was heated by passing the slurry in countercurrent contact with steam in a 6-inch glass-lined steel column packed to a depth of 18 feet with /z-inch 10 Earl saddles. In this operation, the slurry was fed to the top of the column at a rate of 50 pounds per hour, ammonia being withdrawn from the top of the column. The resulting slurry had a pH of 7.6.

After filtration and drying at a temperature of about to C., the product had the following composition:

Percent by weight Chloride 1.47 Free H2O 10.45 Ignition loss 12.49 CaO 1.85 SiOz a Balance pH 8.2 This product is an effective rubber reinforcing pigment.

Example VI Twelve gallons of silica slurry prepared as described in Example V, after the steam treatment, was mixed with 200 cubic centimeters of an aqueous solution of hydrochloric acid containing 32 percent by weight of HCl. The resulting mixture was allowed to digest at a temperature of 30 C. for 16 hours. Thereafter, the slurry was filtered and the resulting silica dried. This product, when incorporated in rubber according to standard methods, yielded results which were superior to those obtained using silica prepared according to Example V.

It will be understood that silica pigments which come within the purview of this invention can be prepared by other processes. For example, silica can be precipitated by reaction of ammonium chloride with sodium silicate under specific conditions which are controlled and adjusted so as to produce a pigment of the character herein contemplated. Moreover, sodium silicate may be reacted with acids, notably carbonic acid, in order to produce a pigment of the type herein contemplated. The following examples are illustrative:

Example VII A 30-gallon, open-top barrel, provided with an agitator consisting of a vertical shaft driven by a hp. motor and having three 3-inch propellers, was charged with 48 liters of an aqueous solution of sodium silicate,

NazO (SiO2)2 .36

containing 20.3 grams per liter of NazO, about 68 grams per liter of SiOz, and 10 grams per liter of sodium chloride. Carbon dioxide gas, diluted with air to such an extent that the diluted gas had a C02 concentration of about 10 percent by volume, was introduced into the drum through a stainless steel tube with the discharge end of the tube being located below the bottom of the agitator. The rate of introduction of gas was adjusted so that just the theoretical amount of carbon dioxide required to produce sodium carbonate was introduced into, the solution in 24 hours. This carbonation rate was held substantially constant over the carbonation period. The temperature was maintained at 35 C. during carbonation and the mixture continuously agitated.

After the theoretical amount of carbon dioxide (the amount of carbon dioxide sufficient to react with the sodium carbonate and to convert all the N320 content thereof to sodium silicate) had been introduced, the mixture was heated by direct introduction of steam from a -pound steam line to maintain the temperature of the slurry at boiling temperature for a period of about 2 hours. The heated slurry was then filtered and the de- Watered silica dried in an oven at a temperature of 108 C., after which it was micropulverized.

The surface area of the resulting finely divided silica was determined by the standard low temperature nitrogen adsorption method proposed by Brunauer, Emmett, and

The silica pigment as thus produced was incorporated 11 by conventional compounding methods in the following GR-S rubber recipe:

' t a Parts by weight GR-S 100.0

The resulting rubber composition was subjected to various curing times at 280 F. and then testedifor tensile strength and tear strength. It was found that the tensile strength of the vulcanized rubber, after curing for 20 minutes at 280 F, was 2440 p. s. i., and the tear strength was 250 pounds per inch thickness.

In the 'preparation'of the pigment under the above conditions, the presence of the soluble salt is important.

Silica formed under the same conditions but with smaller amounts of sodium chloride was inferior as a rubber pigment, and when the sodium chloride was omitted,

the precipitated silica gelled and was discarded.

Example VIII Twenty-six hundred gallons of sodium silicate solution containing 18 grams per liter of NaCl and 20 grams per liter of NazO as the sodium silicate,'Na2O(SiO2)3.s, was'placed in a 4000-gallon tank. Carbon dioxide gas containing percent by volume of CO2, the balance being nitrogen, was introduced into the solution over a period of 3 hours while holding the solution at 30 C. at a rate sufiicient to react with all of the sodium silicate and convert to percent of the NazO content thereof to bicarbonate. Thereafter, the resulting slurry was boiled for one hour,.filtered, and washed. The filter cake was reslurried and a solution of Al2(SO4)318H2O in quantity sufiicient to introduce into the slurry /2 percent of Al, based upon the weight of the SiOz in the slurry, was added to the slurry. Thereafter, the slurry was stirred briefly and enough hydrochloric acid was added to adjust the pH to 5.7. The resulting slurry was filtered and the filter cake dried.

Example IX A 90-liter autoclave kettle provided with a heating and cooling coil, an agitator, and a metal thermometer, was charged with 48 liters of a solution containing 20 grams per liter of sodium chloride and a quantity of sodium silicate sufiicient to cause the solution to contain 20.3 grams per liter of NazO and about 68 grams per liter of SiOz. Essentially pure carbon dioxide was introduced through the bottom of the kettle under the liquid level of the solution at a point about 1 inch below the Cell ter of the agitator. The temperature was maintained at 25 C. during carbonation.

The carbon dioxide was fed to the solution at such a rate as to deliver thetheoretical amount of carbon dioxide thereto in 4 hours and carbonation was continued at this rate for 5 hours, thus providing a 25 percent excess of COz over that theoretically requiredto produce the carbonate.

After 5 hours of carbonation, a sample of the slurry (designated sample A in the table below) was taken out, the pigment filtered, washed twice with water, reslurried, and the pH of the slurry adjusted'to 7.3 with hydrochloric acid. Thereafter, the pigment was washed until the filtrate was substantially chloride-free. I

The slurry remaining in the kettle was boiled for 1- hour and two samples of the boiled slurry removed-from the kettlel One of these samples (designated sample B in the table below) was washed with water alonerwhi'le the other. (sample-C) was reslurried and'the pH of the slurry adjusted to 7.2 with hydrochloric acid. The acidified pigment was then washed substantially chloride-free with water.

j The slurry remaining in the kettle was maintained under a carbon dioxide atmosphere with agitation'for an additional 2 hours. The carbon dioxide pressure was maintained from about 2 to 5 pounds per square inch gauge. This treatment of the slurry with carbon dioxide under pressure reduced the pH of the slurry somewhat. A sample of the thus treated slurry (sample D) was recovered by filtration and washed with tap water. All of the samples A to D were dried at C. in a forced-draft laboratory oven, were micropulverized, and then compounded in rubber in accordance with the procedure of Example VII, after which the tensile and tear strengthsof the samples were determined. Table III below lists the surface areas of the different samples before incorporation in the rubber, and the tensile and tear strengths of the rubber compositions in which the respective samples were incorporated.

TABLE III Surface Tensile Tear Area Strength Strength Sample N 0. (square (pounds (pounds meters per square per inch) per gram) inch) A 337 3, 280 B. 122 3, 330 C 138 3, 220 400 127 3, 550 320 Example X A 4000-gallon rubber-lined tank equipped with a motordriven turboagitator was charged with a 2700-gallon batch of silicate-salt solution prepared by adding hot concentrated sodium silicate to brine in the ratio of 1:4. The sodium silicate solution was adjusted to contain 20.3 grams per liter of NazO and 17.4 grams per liter of NaCl. The solution, after mixing, was brought to a temperature of 30 C. by heating with live steam, and was then carbonated by bubbling therethrough a gas containing 40 percent CO2 to precipitate silica. The gas heating rate was such as to increase the temperature of the solution about 1 C. per minute, and, when the boiling point had been reached, the solution was boiled for one hour. During heating and boiling, the gas was added at a reduced rate. 0

After standing overnight, the resulting slurry was pumped to a 3-foot by 3-foot wash wheel where the slurry was filtered and the cake washed to remove salt and alkali. The slurry fed to the wash wheel was kept hot by steam-heated coils, and hot condensate was used as wash water. The washed filter cake was reslurried, then again filtered, then reslurried and returned to the precipitation tank where the alkali therein was neutralized with 10 N HCl to reduce the NazO content to 0.5 gram per liter. The salt content of the neutralized slurry was 4.0 grams'NaCl per liter, and the pH was 7.2. The slurry contained0.43 pound per gallon of solids. The slurry was then pumped to a 2-foot by 4-foot diameter vacuum filter wheel and filteredJ The filter cake. from this wheel was charged to a 3-foot by 25-foot steam-heated rotary drier and .dried to a moisture: content of 4 percent to 6 percent,.after whichthe dried material was pulverized ina No. 1 micropulverizer. The silica pigment 'as thus prepared was incorporated in a GR-S rubber formula. asjfollows; f

A rubber sample as thus compounded was cured for 20 minutes at 280 F., and gave a tensile strength of 3280 pounds per square inch and a tear strength of 250 pounds per inch.

Example XI A 90-liter autoclave kettle provided with a heating and cooling coil, an agitator, and a metal thermometer, was charged with 12,850 grams of sodium silicate solution containing 976 grams of NazO and 3,115 grams of SiOz. The solution was diluted to 48 liters total volume and the temperature raised to 95 C. The solution was carbonated with 100 percent CO2 and a carbonation rate was used such as to introduce the theoretical amount of CO2 in about 30 minutes. Carbonation was continued at this rate for about 1 hour, at the end of which time the pH of the slurry was 9.85.

The resulting slurry was filtered and washed twice with hot tap water. The filter cake was reslurried and adjusted to a pH of 6.75 by adding 400 cubic centimeters of 3.5 N HCl thereto. The acidified slurry was then filtered and the filter cake washed nearly chloride-free with hot tap water, after which the precipitate was dried at 105 C. in a forced-draft laboratory oven, then micropulverized, air-conditioned, analyzed, and compounded in rubber as previously described. The finished pigment had a pH of 8.2 and contained 0.61 percent sodium. its 13. E. T. surface area was 148 square meters per gram. A vulcanized rubber sample incorporating the pigment exhibited a tensile strength of 2710 pounds per square inch and a tear strength of 260 pounds per inch.

Example XII A series of experiments was performed in which silica was precipitated with ammonium chloride in a 30-gallon, rubber-lined, open-top barrel provided wtih three 3-inch propellers on the shaft of a A H. P. laboratory stirrer. Where necessary, the solutions were heated by direct steam from a 140 pounds per square inch steam line.

In the performance of these tests, sodium chloride was first dissolved in water and then sodium silicate, having the composition NazO (Si02)3.3e, was added. The amount of NazO in the sodium silicate was maintained constant at488 grams. The SiOz content of the solution was approximately 1640 grams, with very slight variation. The mixture was brought to the desired reaction temperature and the total volume of the solution was made up by dilution with water to 48 liters. The concentration of the NazO, therefore, was 10.15 grams per liter, and the SiOz concentration was about 33.8 to 34.3 grams per liter in all experiments. After a waiting period of about 30 minutes, the ammonium chloride solution was This ammonium chloride solution contained the following approximate composition:

Grams per liter Free ammonia asNHa 17.5

Fixed ammonia in ammonium chloride computed as NPR 56.1 Total chlorine 160.2 Fixed ammonia calculated as ammonium chloride 176.1 Sodium chloride 71.8

The pH of this solution was approximately 10.3.

The above analysis of the ammonium chloride solution is approximate and slight variations occurred over the range of experiments, usually not in excess of about plus or minus 3 or 4 percent.

In the performance of the process, the sodium chloridesodium silicate solution was allowed to stand for 30 minutes and thereupon the ammonium chloride solution was added in amount equal to 108 percent of the equivalent of the NazO, i. e., 910 grams of NHiCl. The ammonium chloride solution was added by means of a variable speed metering pump. Following the addition, the resulting slurry was brought to boiling within about 5 minutes by direct heating with steam. This heating was continued and ammonia distilled from the slurry until the alkalinity of the raw slurry had only a slight phenolphthalein reaction. The heating period varied between 60 and 90 minutes. The hot slurry was then filtered and the filtercake was washed free'from chloride ion with cold water. In the first washing, about 400 cc. of 3.5 N hydrochloric acid was used. Thereafter, the filter cake was washed free from chloride ion with water. The hydrochloric acid treatment had the effect of eliminating all remaining alkali, and therefore all of the finished product had a pH between 4 and 6.5. The filter cakes were driedat 110 C., the pigment micropulverized, and the surface area of the various pigments was measured, being found to range between to 200 square meters per gram. 7

Example XIII In each of a series of large-scale experiments, a sodium silicate solution having the composition NazO(SiO2)s.ss, and containing the amount of sodium silicate equivalent to the NazO content set forth in the following table, was placed in a 4000-gallon tank. The NaCl content and temperature of the solution were as stated in the table.

,Amrnonium chloride solution containing about 44.8 grams per liter of fixed N'Ha as NHiCl, 73.9 grams per liter of NaCl, and 16.2 grams per liter of free NHa was added (with vigorous agitation, using a 25 hp. turboagitator) while maintaining the temperature of the mixture substantially constant. After precipitation, the slurry was heated to about 90l00 C. in a still to drive ofi evolved NHa, and thereafter the suspension was allowed to condition for at least /2 hour at a temperature above C. Following this, the slurry was filtered and the resulting product was recovered, dried at about 1l0-125 C. in a rotary drier, and incorporated in a GR-S rubber composition for testing according to standard methods.

The results are as follows:

added.

TABLE IV Amount of Am- NaGl Content NazO Content Surface Area monium Chlo- Final pH of Sodium of Sodium Silicate Temp, Time of of Product, Experiment N0. ride Solution of pi Silicate Solu- Silicate Solu- Solution 0. Addition Square Meters a ded ment tion (grams tion (grams (Gallons) (Minutes) per gram (Gallons) per liter) per liter) 1 In this test, of the amount of ammonium chloride required was added in 3% hours.

about 15 minutes.

The balance was added rapidly within *15 As shown by the above disclosure, the recovery of silica from an aqueous slurry formed by acidification of a solution or slurry of a metal silicate to solubilize the metal produces a slurry having a pH below 5. Adjustment of the slurry pH above enables rapid settling and/or filtration o f'the slurry. 7 A 7 V H Silica prepared according to this invention hasa bulk density, of about 5 to 10 pounds per cubic foot. When milled into a rubber composition, some dusting tends to occur.

According fo a further embodiment of this invention,

this dusting may be avoided by compacting the silica into cakes of convenient size and increased bulk density. Thus, it has been found that this silica may be compressed to a bulk density of about to 30 pounds, preferably to pounds, per cubic foot, by compacting the silica under a pressure of about 125 to 200 pounds per square inch. Compacting to a bulk density above about pounds percubic foot is objectionable because the product becomes difficult to mill into rubber without pregrinding.

This compressing may be'effected simply by pouring the silica into a mold and applying pressure to the silica in a mold, thus producing a bonded cake of any convenient volume, for example, one or more cubic feet. The free water in the cake probably aids to facilitate the bonding of the particles under the applied pressure.

. Example XIV Seventeen thousand gallons of a sodium silicate solution is placed in a 50,000-gallo11 tank. This solution contains the sodium silicate Na2O(SiO2)a.3 in amount sufiicient to establish an NazO concentration of 20.3 grams per liter. This solution contains no sodium chloride except that minor amount (less than 0.08 percent) usually present in commercial sodium silicate. The solution is held at a temperature of 167 F.:0.5. Carbon dioxide gas containing 10.0 to 10.8 percent of CO2, the balance being nitrogen and air, is introduced into the solution at a gas temperature of 115 to 145 F. at a rate sufiicient to provide 1250 cubic feet of the carbon dioxide gas per minute (measured at 760 millimeters pressure and 0 C.). This gas is introduced directly under a turboagitator in a manner to achieve uniform distribution of gas, and the mixture is vigorously agitated. Carbon dioxide introductionis continued at this rate for 8 /2 hours, at which time about 120 to 140 percent of the theoretical amount Of'COZ has been introduced. Aftef'this'pe iiod-of 8% hours, the'r'atof introduction of'carbon dioxide is reduced to'400 eusiereetpermamreanarne solution is boiled for 1% hours. The slurry isthen washed and the silica is recovered. The silica has a surface area of 140 to 150 square meters per gram.

Although the presentinvention has been described with reference to the'specific details'ofsuchan embodiment, it is not intendedthat such details shall be regarded as limiting the scope of the present invention except insofar as such limitations appear in the, accompanying claims.

This application is a continuation-in-part of my copending application Serial No. 63,205, filed December 2, 1948 (now abandoned), and is a division of my copending application Serial No. 283,721, filed April 22, 1952.

What is claimed:

' 1. Vulcanized composition containing a rubber of the group consisting of natural rubber and synthetic rubbery polymers of conjugated dienes and having dispersed therein from about 5 to 100 parts by weight of a finely divided silica pigment to about 100 parts by weight of said rubber, said finely divided silica pigment containing at least 90 percent by weight of SiOz measured on the anhydrous basis, bound water in the proportion of one mole of water per 3 to 9 moles of SiOz, the free water content of said pigment being up to 10 percent by weight; said pigment being in the form of porous flocs and having a surface area of to 200 square meters per gram and an average ultimate particle size of 0.015 to 0.05 micron.

2. The rubber of claim 1 in which the amount of said pigment is 40 to parts by weight per parts by weight of rubber.

3. The product of claim 1 wherein the rubber is a rubbery butadiene-styrene copolymer rubber.

4. The product of claim 1 wherein the rubber is natural rubber.

5. The product of claim 1 wherein the rubber is a butadiene-acrylonitrile copolymer.

6. The product of claim 1 wherein the NazO content of the pigment does not exceed about 1.75 percent.

No references cited. 

1. VULCANIZED COMPOSITION CONTAINING A RUBBER OF THE GROUP CONSISTING OF NATURAL RUBBER AND SYNTHETIC RUBBERY POLYMERS OF CONJUGATED DIENES AND HAVING DISPERSED THEREIN FROM ABOUT 5 TO 100 PARTS BY WEIGHT OF A FINELY DIVIDED SILICA PIGMENT TO ABOUT 100 PARTS BY WEIGHT OF SAID RUBBER, SAID FINELY DIVIDED SILICA PIGMENT CONTAINING AT LEAST 90 PERCENT BY WEIGHT OF SIO2 MEASURED ON THE ANHYDROUS BASIS, BOUND WATER IN THE PROPORTION OF ONE MOLE OF WATER PER 3 TO 9 MOLES OF SIO2, THE FREE WATER CONTENT OF SAID PIGMENT BEING UP TO 10 PERCENT BY WEIGHT; SAID PIGMENT BEING IN THE FORM OF POROUS FLOCS AND HAVING A SURFACE AREA OF 60 TO 200 SQUARE METERS PER GRAM AND AN AVERAGE ULTIMATE PARTICLE SIZE OF 0.015 TO 0.05 MICRON. 